Trailers that are used to haul boats over land usually include a frame mounted on wheels, a tongue extending forwardly from the frame that connects to a hitch ball of the towing vehicle, and parallel boat support boards, known as bunk boards, that are mounted to the frame. The trailer is backed down a boat launching ramp toward a body of water until the aft ends of the bunk boards are submerged in the water. A winch mounted over the tongue of the trailer is connected to the bow of the boat and is used to pull the boat from the water onto the bunk boards. The bunk boards are arranged so that their load bearing surfaces generally conform to the shape of the boat hull and are sometimes mounted on pivots so that they can tilt slightly to have their load bearing surfaces positioned in flat abutment with the curved hull of the boat.
When the boat is in the water and the boat trailer is backed down a ramp to retrieve the boat, its wheels and the aft ends of the bunk boards become submerged in the water so that the boat hull can engage the aft ends of the bunk boards at a position beneath the surface of the water. The momentum of the boat moving toward the trailer causes the boat to first engage the aft ends of the bunk boards that are not readily visible to the driver of the boat, and then the momentum of the boat usually carries the boat partially up the bunk boards toward the forward portion of the trailer, depending upon the skill of the driver. In some instances, there will be a current in the water, such as water adjacent a river bank, that tends to move the boat sideways as the boat is approaching the trailer and as the boat first engages the bunk boards. If the trailer is driven down the ramp a distance that makes the bunk boards deep in the water, the driver of the boat might misjudge the position of the bunk boards and the boat might not be properly aligned with the trailer as the boat approaches the trailer.
It would be helpful if the driver of the boat could accurately determine the positions of the submerged ends of the bunk boards and the position of the approaching bow of the boat with respect to the bunk boards as the boat approaches the trailer for mounting on the trailer.